Abstract

A novel in-fiber modal interferometer based on a long period grating (LPG) inscribed in a two-mode all-solid photonic bandgap fiber (AS-PBGF) is presented. After inserting a small piece of the AS-PBGF into two sections of standard single-mode fiber (SMF) via being spliced slight core offset, LPG is inscribed in the AS-PBGF. The LPG is especially designed to realize the coupling between two core modes of LP01 and LP11 in the AS-PBGF. Two core modes LP01 and LP11 of the AS-PBGF are excited firstly at the input spliced point and actualized energy exchange when they pass through the LPG. Then the two beams will interfere at the output spliced point to form a high-contrast in-fiber modal interferometer. The proposed interferometer has some advantages such as configuration compact, high interference contrast and the wavelength spacing well controlled by changing the position of the LPG without changing the total length of AS-PBGF.

(a)The measured transmission spectrum of the interferometer. The insets are modal distribution for the grating with pitch 620um measured at (i)1550.00 nm, (ii)1582.20 nm, (iii)1597.27nm,and (iv)1609.15nm . (b)Comparison of the measured interference spectra with different length of AS-PBGF. Black and red curves are the measured spectrum of the interferometer for ΔL1=8.3 cm and ΔL2=7.0 cm, respectively.

Comparison of the spectra of interferometers made up of LPGs with different grating pitch. (a) phase-matching curve of AS-PBGF, spectra of interferometer for the LPGs with a pitch of (b) 610 μm and (c) 620 μm.